Conventional systems in a vehicle for measuring ambient air temperature include a sensor placed on the vehicle in a location exposed to ambient air. Any location on the vehicle, however, is subject to heat generated by the vehicle engine, exhaust, transmission, etc., which could influence a temperature measurement. When the vehicle is stationary or moving slowly, for example, a boundary layer of hot air exists around the engine compartment and exhaust components, and ambient air temperature measurement is influenced by the heat in this boundary layer.
Various methods for reducing or correcting for the effect of vehicle-generated heat are known. U.S. Pat. No. 5,813,765 to Peel et al. includes placing the temperature sensor on the radio antenna to locate it as far as possible from vehicle heat sources. U.S. Pat. No. 7,387,437 to Brown et al. discloses using multiple sensors placed at different locations on the vehicle, comparing the readings from the multiple sensors, and applying a correction factor generated according to vehicle speed and other factors. U.S. Pat. No. 4,770,543 to Burghoff et al. discloses a method including monitoring vehicle speed and using a temperature measured when speed exceeds a threshold, and, when speed drops below the threshold, storing and using a last temperature measurement before the speed dropped below the threshold.
The invention improves on the art in proposing a method and system for measuring ambient air outside a vehicle that overcomes deficiencies in the art. According to the invention, a temperature sensor is mounted in a flow path of an air blowing or moving device, such as the engine radiator fan. A controller is configured to monitor at least one vehicle parameter, compare the vehicle parameter to a reference parameter, and responsive to the comparison, control the air blowing device and make an ambient air temperature reading.
According to the invention, the vehicle parameter may be a vehicle speed value, and the air blowing device is controlled to be activated to move ambient air across the sensor if the vehicle speed value is below a road speed reference value stored in a controller memory.
According to another aspect of the invention, the method includes the steps of monitoring the vehicle speed for a measured interval after taking the temperature reading, and, if the determined road speed remains below the road speed reference value during the interval, further including activating the device to move ambient air across the sensor, taking a temperature reading with the sensor, and holding the temperature reading as the current ambient air temperature if the temperature reading is lower than the stored temperature value.
According to another aspect of the invention, the sensor is mounted in an air flow path of a vehicle radiator fan, and the step of controlling the device to move ambient air across the sensor comprises activating the radiator fan.
Alternatively, an air blowing device to move air across the sensor may be provided if locating the sensor in the radiator fan flow path is not convenient or desired.
According to another aspect of the invention, the method includes applying a correction to the temperature reading and storing the corrected temperature reading as the ambient air temperature. The correction may compensate for the effect of heat not removed by the activated fan, and may constitute a subtraction of 2 degrees Celsius.
The method also includes the step of communicating the current ambient air temperature to vehicle components, for example, by broadcasting a temperature signal on the CAN bus. The Engine Management System and other controllers can then make use of the temperature information to perform various functions.
According to the invention, the method includes taking a plurality of temperature readings during a predetermined time duration while the fan is activated and storing a lowest temperature reading as the current ambient air temperature.
According to another aspect of the invention, if the determined road speed is above the road speed reference value, for example 50 kph, for at least a predetermined time duration, the method includes continuously taking temperature readings as the ambient air temperature. The predetermined time duration may be set at 90 seconds to allow sufficient air flow over the temperature sensor to remove enough engine heat for an accurate ambient air temperature reading.
According to yet another aspect of the invention, if the determined road speed is above a second road speed reference value, for example 80 kph, which is higher than the first road speed reference value, the method includes immediately continuously taking temperature readings and storing a lowest value as the ambient air temperature.
According to another aspect of the invention, the method includes monitoring the engine temperature, and upon determining that the engine temperature is below a reference temperature, for example, 50 degrees Celsius, which indicates a cold engine start, taking a temperature reading with the sensor, and, holding the temperature reading as the current ambient air temperature.
According to yet another aspect of the invention, the method includes monitoring the engine temperature and if the engine temperature is above the reference temperature, for example, 50 degrees Celsius, which indicates a warm engine or may indicate a warm engine start, holding the last held temperature reading as the current ambient air temperature for a measured interval, activating the device to move ambient air across the sensor after the measured interval, taking a temperature reading with the sensor and, holding the temperature reading as the current ambient air temperature if the temperature reading is lower than the held temperature value. The measured interval for holding the last held temperature may be 60 minutes, for example.